Subterranean well pressure and temperature measurement

CROSS-REFERENCE TO RELATED APPLICATION

0001The present application claims the benefit under 35 USC §119 of the filing date of international application serial no. PCT/US02/23272, filed Jul. 23, 2002, the entire disclosure of which is incorporated herein by this reference.

GOVERNMENT LICENSE RIGHTS

0002The U.S. Government has a paid-up license in this invention and right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. CRADA# NCRADA-NRL-99-249 awarded by The Naval Research Laboratory.

BACKGROUND

0003The present invention relates generally to operations performed and equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides pressure and temperature measurement in such wells.

0004Pressure and temperature measurement in wells has generally been accomplished using pressure and temperature transducers which utilize separate pressure and temperature sensors. For example, a transducer may include a piezoelectric pressure sensor and a thermocouple temperature sensor.

0005Unfortunately, however, most methods of measuring pressure are sensitive to changes in temperature. If the temperature changes relatively rapidly, as happens many times during typical well operations, such as drill stem testing, gravel packing, etc., then it may be very difficult to accurately measure pressure in the well. For example, for the transducer described above, the thermocouple and the piezoelectric crystal may have different temperature change response rates and/or may be located in different positions in the transducer, so that the temperature indicated by the thermocouple is not necessarily the temperature of the piezoelectric crystal.

0006Therefore, it would be highly desirable to provide a method of measuring pressure in a well in which accuracy of the measurement is improved. The ability to eliminate the contribution of temperature change to the pressure measurement would be very advantageous.

SUMMARY

0007In carrying out the principles of the present invention, in accordance with an embodiment thereof, a sensor system is provided. The sensor system is an advance in the art in that it eliminates one or more problems in prior sensor systems, and/or performs more efficiently than prior systems.

0008Methods of measuring pressure in a well are also provided by the present invention. In one example, a pressure measuring method includes the steps of applying a pressure differential across a structure positioned in a well, and applying a temperature change to the structure. A strain in the structure induced by the pressure differential and the temperature change is detected using one sensor. A different strain, also induced by the pressure differential and the temperature change, is detected using another sensor.

0009A predetermined mathematical relationship exists between the pressure differential and the strains. Therefore, the pressure differential may be readily known when the first and second strains are known. In this example, the strains detected by the sensors may each include the same contribution due to strain induced by the temperature change in the structure.

0010In another example of a method of measuring pressure in a well provided by the invention, the method includes the steps of applying a pressure differential and a temperature change to a structure positioned in a well, detecting a strain in the structure induced by the pressure differential and the temperature change using one sensor, and detecting another strain in the structure induced by the temperature change using another strain sensor.

0011The latter strain sensor does not detect strain induced in the structure by the pressure differential. However, a predetermined mathematical relationship does exist between the pressure differential and the strains. In this example, the difference between the strains equals the strain induced in the structure by the pressure differential, since each of the strains includes the same contribution due to the temperature change.

0012In yet another method of measuring pressure in a well, the method includes the steps of applying a pressure differential across a structure positioned in the well, detecting a strain in the structure in one direction using a strain sensor, detecting a different strain in the structure in another direction using another strain sensor, and calculating the pressure differential using a predetermined mathematical relationship between the pressure differential and the strains.

0013Specific examples are described below of various sensor systems provided by the invention. In one example, a sensor system includes a structure having multiple portions. A pressure differential is applied across one portion, the pressure differential existing between surfaces formed on the portion.

0014Strain sensors detect strain in the structure induced by a temperature change in the well. One strain sensor detects strain in the one portion induced by the pressure differential. Another strain sensor detects strain in another portion of the structure due to the temperature change, but does not detect strain in the other portion induced by the pressure differential.

0015In another example, a sensor system is described which includes a structure having a pressure differential applied across a membrane of the structure. The pressure differential exists between well pressure applied to one side of the membrane and another pressure applied to another side of the membrane. Strain sensors detect strain in the membrane induced by the pressure differential and by a temperature change in the well. One strain sensor detects strain in the membrane in a first direction, and the other strain sensor detects strain in the membrane in a second direction.

0016In still another example, a sensor system is provided which includes a tubular structure having a pressure differential applied across its inner and outer surfaces. The pressure differential exists between well pressure applied to one of the inner and outer surfaces and another pressure applied to the other of the inner and outer surfaces. Strain sensors detect strain in the structure induced by the pressure differential and by a temperature change in the well. One strain sensor detects strain in the structure in a first direction, and another strain sensor detects strain in the structure in a second direction.

0017In a further example of a sensor system provided by the invention, the sensor system includes a structure in which strain is induced in response to a pressure differential in a well and strain sensors attached to the structure and detecting strain in the structure when the pressure differential exists in the well. One strain sensor detects a strain in a direction in the structure, and another strain sensor detects another strain in another direction in the structure.

0018In each of the above examples, at least one of the strain sensors may be a fiber optic sensor. The fiber optic sensor may comprise an interferometer or fiber grating. Any of a wide variety of fiber optic sensors may be used.

0019These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0020<figref idref="DRAWINGS">FIG. 1</figref> is a schematic view of a method embodying principles of the present invention;

0021<figref idref="DRAWINGS">FIG. 2</figref> is a schematic cross-sectional view of a sensor system embodying principles of the invention;

0022<figref idref="DRAWINGS">FIG. 3</figref> is a graph representative of reflectivity vs. wavelength for fiber optic sensors of the sensor system of <figref idref="DRAWINGS">FIG. 2</figref>;

0023<figref idref="DRAWINGS">FIG. 4</figref> is a schematic cross-sectional view of a second sensor system embodying principles of the invention;

0024<figref idref="DRAWINGS">FIG. 5</figref> is a cross-sectional view of the second sensor system, taken along line <b>5</b>—<b>5</b> of <figref idref="DRAWINGS">FIG. 4</figref>; and

0025<figref idref="DRAWINGS">FIG. 6</figref> is a schematic cross-sectional view of a third sensor system embodying principles of the invention.

DETAILED DESCRIPTION

0026Representatively illustrated in <figref idref="DRAWINGS">FIG. 1</figref> is a method <b>10</b> which embodies principles of the present invention. In the following description of the method <b>10</b> and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention.

0027In the method <b>10</b>, a sensor system <b>12</b> is positioned in a well for measuring pressure and temperature during operations in the well. As depicted in <figref idref="DRAWINGS">FIG. 1</figref>, the sensor system <b>12</b> is conveyed into the well attached to an item of equipment <b>14</b>, such as a drill stem testing tool. However, it is to be clearly understood that the sensor system <b>12</b> may be otherwise positioned in the well, and may be used in well operations other than drill stem testing, in other methods embodying principles of the invention.

0028Lines <b>16</b> extend from the sensor system <b>12</b> to surface data acquisition equipment <b>18</b>. The lines <b>16</b> may be any type of data transmission lines, such as electrical lines, fiber optic lines, hydraulic lines, etc., and the lines may also perform other functions, such as power source, control, etc. Alternatively, data transmission and any of the other functions of the lines <b>16</b> may be provided by wireless means, such electromagnetic, acoustic, pressure pulse or other type of telemetry.

0029Preferably, the sensor system <b>12</b> includes one or more fiber optic sensors, and so the lines <b>16</b> in the method <b>10</b> may conveniently be fiber optic lines extending into the well between the sensor system and the data acquisition equipment <b>18</b>. In that case, the equipment <b>18</b> could include the appropriate means for optical communication, such as one or more lasers, photoelectric detectors, etc., which are known to those skilled in the art. However, other configurations are available to transmit data between the sensor system <b>12</b> and the data acquisition equipment <b>18</b>.

0030Referring additionally now to <figref idref="DRAWINGS">FIG. 2</figref>, an example is representatively illustrated of a sensor system <b>20</b> which may be used in the method <b>10</b>. The sensor system <b>20</b> includes a tubular structure <b>22</b> in which strain is induced by a pressure differential and temperature change in the well. The strain is detected by strain sensors <b>24</b>, <b>26</b>.

0031Temperature change induces strain in the structure <b>22</b> in the sense that temperature change causes a dimensional change which is detected as a strain in the structure by the strain sensors <b>24</b>, <b>26</b>.

0032As depicted in <figref idref="DRAWINGS">FIG. 2</figref>, the structure <b>22</b> is preferably a closed-end hollow cylinder, whose length is at least several times its outer diameter, and whose wall thickness is relatively thin. In this manner, hoop and axial strain in the structure <b>22</b> due to a pressure differential between the interior and exterior surfaces of the structure may be readily calculated using known mathematical relationships, and contributions due to edge effects, discontinuities, etc. are reduced or eliminated. However, it should be clearly understood that principles of the invention may be incorporated into sensor systems in which pressure responsive structures have other shapes, and in which strains have other mathematical relationships to pressure applied to the structure.

0033For the closed-end tubular structure <b>22</b> depicted in <figref idref="DRAWINGS">FIG. 2</figref>, axial strain due to a pressure differential across interior and external surfaces of the structure may be calculated using the following formula: <maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mrow><msub><mi>ɛ</mi><mi>zz</mi></msub><mo>=</mo><mrow><mfrac><mn>1</mn><mi>E</mi></mfrac><mo>⁢</mo><mrow><mo>(</mo><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><mrow><mrow><mo>(</mo><mrow><mrow><mn>2</mn><mo>⁢</mo><mi>υ</mi></mrow><mo>-</mo><mn>1</mn></mrow><mo>)</mo></mrow><mo>⁢</mo><msub><mi>P</mi><mi>i</mi></msub></mrow><mo>+</mo><mrow><mi>υ</mi><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><msub><mi>P</mi><mi>o</mi></msub></mrow></mrow><mo>)</mo></mrow><mo>⁢</mo><msubsup><mi>r</mi><mi>i</mi><mn>2</mn></msubsup></mrow><mo>-</mo><mrow><mn>2</mn><mo>⁢</mo><mi>υ</mi><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><msub><mi>P</mi><mi>o</mi></msub><mo>⁢</mo><msubsup><mi>r</mi><mi>o</mi><mn>2</mn></msubsup></mrow></mrow><mrow><msubsup><mi>r</mi><mi>i</mi><mn>2</mn></msubsup><mo>-</mo><msubsup><mi>r</mi><mi>o</mi><mn>2</mn></msubsup></mrow></mfrac><mo>)</mo></mrow></mrow></mrow></math></maths>

0034Hoop strain due to the pressure differential may be calculated using the following formula: <maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mrow><msub><mi>ɛ</mi><mi>θθ</mi></msub><mo>=</mo><mrow><mfrac><mn>1</mn><mi>E</mi></mfrac><mo>⁢</mo><mrow><mo>(</mo><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><mrow><mrow><mo>(</mo><mrow><mrow><mn>2</mn><mo>⁢</mo><mi>υ</mi></mrow><mo>-</mo><mn>1</mn></mrow><mo>)</mo></mrow><mo>⁢</mo><msub><mi>P</mi><mi>i</mi></msub></mrow><mo>-</mo><mrow><mi>υ</mi><mo>⁢</mo><mstyle><mtext> </mtext></mstyle><mo>⁢</mo><msub><mi>P</mi><mi>o</mi></msub></mrow></mrow><mo>)</mo></mrow><mo>⁢</mo><msubsup><mi>r</mi><mi>i</mi><mn>2</mn></msubsup><mo>⁢</mo><msup><mi>r</mi><mn>2</mn></msup></mrow><mo>-</mo><mrow><mrow><mo>(</mo><mrow><mrow><mrow><mo>(</mo><mrow><mi>υ</mi><mo>-</mo><mn>1</mn></mrow><mo>)</mo></mrow><mo>⁢</mo><msub><mi>P</mi><mi>o</mi></msub><mo>⁢</mo><msup><mi>r</mi><mn>2</mn></msup></mrow><mo>+</mo><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mi>υ</mi></mrow><mo>)</mo></mrow><mo>⁢</mo><mrow><mo>(</mo><mrow><msub><mi>P</mi><mi>i</mi></msub><mo>-</mo><msub><mi>P</mi><mi>o</mi></msub></mrow><mo>)</mo></mrow><mo>⁢</mo><msubsup><mi>r</mi><mi>i</mi><mn>2</mn></msubsup></mrow></mrow><mo>)</mo></mrow><mo>⁢</mo><msubsup><mi>r</mi><mi>o</mi><mn>2</mn></msubsup></mrow></mrow><mrow><msup><mi>r</mi><mn>2</mn></msup><mo>⁡</mo><mrow><mo>(</mo><mrow><msubsup><mi>r</mi><mi>o</mi><mn>2</mn></msubsup><mo>-</mo><msubsup><mi>r</mi><mi>i</mi><mn>2</mn></msubsup></mrow><mo>)</mo></mrow></mrow></mfrac><mo>)</mo></mrow></mrow></mrow></math></maths>

0035It will be readily appreciated that, if the dimensions of the structure <b>22</b> are known, appropriate values for Young's modulus (E) and Poisson's ratio (υ) are known for the corresponding structure material, and the axial and hoop strains (ε_zz, ε_θθ) are known from the strain sensors <b>24</b>, <b>26</b>, then the pressure differential across the structure may be readily calculated using the above mathematical relationships.

0036If a temperature change in the well occurs, the strains as detected by the strain sensors <b>24</b>, <b>26</b> may vary. Contraction or expansion of the structure <b>22</b> induced by the temperature change will be detected as strain in the structure by the strain sensors <b>24</b>, <b>26</b>. However, since this contraction or expansion is isotropic in the structure, the same strain contribution due to the temperature change will be detected by each of the strain sensors <b>24</b>, <b>26</b>. Therefore, this contribution due to the temperature change may be conveniently mathematically eliminated from the strains detected by the strain sensors <b>24</b>, <b>26</b>.

0037Note that the strain sensors <b>24</b>, <b>26</b> are preferably positioned very close to each other on the structure <b>22</b>, so that they each sense strain due to the same temperature changes, i.e., they are attached to portions of the structure at the same temperature. It may be preferable to position each of the strain sensors <b>24</b>, <b>26</b> at the same longitudinal distance between opposite ends of the structure <b>22</b>.

0038As depicted in <figref idref="DRAWINGS">FIG. 2</figref>, the strain sensor <b>24</b> is oriented longitudinally relative to the tubular structure <b>22</b> and the strain sensor <b>26</b> is oriented circumferentially relative to the structure. The strain sensor <b>24</b> is used to detect axial strain in the structure <b>22</b> and the strain sensor <b>26</b> is used to detect hoop strain in the structure.

0039The strain sensors <b>24</b>, <b>26</b> are orthogonal relative to each other and are positioned approximately midway between opposite ends of the structure <b>22</b>. For clarity of illustration in <figref idref="DRAWINGS">FIG. 2</figref>, the sensors <b>24</b>, <b>26</b> are depicted on the same lateral side of the structure <b>22</b> and offset longitudinally with respect to each other. However, preferably the sensors <b>24</b>, <b>26</b> are each centered at the same longitudinal position on the structure <b>22</b>, but offset radially with respect to each other by 180° (i.e., on opposite lateral sides of the structure <b>22</b>). Of course, strain sensors may be otherwise oriented and otherwise positioned, and may detect strains in other directions, without departing from the principles of the invention.

0040The strain sensors <b>24</b>, <b>26</b> are depicted in <figref idref="DRAWINGS">FIG. 2</figref> as being attached to an outer side surface of the structure <b>22</b> and positioned in an annular space <b>28</b> between the structure and an outer housing <b>30</b>. Preferably, the annular space <b>28</b> is at atmospheric pressure. Pressure in the well is admitted into an interior bore <b>32</b> of the structure <b>22</b>.

0041Thus, the pressure differential across the structure <b>22</b>, which causes axial and hoop strain in the strain sensors <b>24</b>, <b>26</b>, is the difference between well pressure in the bore <b>32</b> and atmospheric pressure in the annulus <b>28</b>. However, it should be understood that, in other sensor systems incorporating principles of the invention, the pressure differential could be due to other pressures acting on the structure <b>22</b> in other ways. For example, the annulus <b>28</b> could be at a pressure other than atmospheric pressure, the well pressure could be applied external to the structure <b>22</b>, etc.

0042As used herein, the term “well pressure” refers to a pressure to which the sensor system <b>20</b> is exposed in a well, and which it is desired to measure. Thus, well pressure may be a pressure in the interior of a wellbore, a pressure from the exterior of the wellbore, pressure in an item of equipment in a well, etc.

0043Where the strain sensors <b>24</b>, <b>26</b> are fiber optic sensors, a fiber optic line <b>34</b> may be used for communication. Either or both of the strain sensors <b>24</b>, <b>26</b> may be an interferometric fiber optic sensor in which reflection or absorption of specific light wavelengths varies depending on the strain experienced by the sensor. For example, the sensor may include a fiber Bragg grating, a pi-shifted fiber Bragg grating, a long period grating, a co-located fiber Bragg grating and pi-shifted fiber Bragg grating, a fiber Bragg laser, an intrinsic or extrinsic Fabry-Perot interferometer, a Michelson interferometer, a Mach-Zehnder interferometer, a fiber ring resonator, etc. Of course, other types of strain sensors may be used, such as metal foil strain gauges, etc.

0044Referring additionally now to <figref idref="DRAWINGS">FIG. 3</figref>, a graph <b>38</b> is representatively illustrated of reflectivity vs. wavelength for two types of fiber optic strain sensors. A plot <b>36</b> for a uniform fiber Bragg grating is shown as a dashed line in the graph <b>38</b>, and a plot <b>40</b> for a pi-shifted fiber Bragg grating is shown as a solid line in the graph.

0045Note that the uniform fiber Bragg grating plot <b>36</b> has a peak <b>42</b> in reflectivity at a wavelength of 1550 nm, while the pi-shifted fiber Bragg grating has a peak <b>44</b> in transmission (a dip in reflectivity) at that wavelength. As the length of the sensor changes, the wavelength at which the peaks <b>42</b>, <b>44</b> occur changes. Thus, these fiber optic sensors accurately measure strain by reflecting and transmitting certain light wavelengths. The pi-shifted fiber Bragg grating may be preferred over the uniform fiber Bragg grating since its peak <b>44</b> is narrower and more well defined.

0046Referring additionally now to <figref idref="DRAWINGS">FIG. 4</figref>, another sensor system <b>50</b> embodying principles of the invention is representatively illustrated. The sensor system <b>50</b> includes an outer housing <b>52</b> in which two chambers <b>54</b>, <b>56</b> are separated by a membrane <b>58</b>. As depicted in <figref idref="DRAWINGS">FIG. 4</figref>, the chambers <b>54</b>, <b>56</b> and membrane <b>58</b> are integrally formed, but they could instead be formed of various elements and materials.

0047The membrane <b>58</b> is illustrated as being circular-shaped and having a uniform wall thickness. However, sensor systems incorporating principles of the invention may include membranes having other shapes and configurations. Preferably, the membrane <b>58</b> has a shape and configuration for which a predetermined mathematical relationship exists between strain detected by one strain sensor <b>60</b> and strain detected by another strain sensor <b>62</b> attached to the membrane.

0048The strain sensors <b>60</b>, <b>62</b> may be similar to the strain sensors <b>24</b>, <b>26</b> described above. In particular, the strain sensors may be fiber optic strain sensors.

0049For the membrane <b>58</b> of the sensor system <b>50</b>, the strain sensor <b>60</b> is attached centrally on the membrane, and the strain sensor <b>62</b> is attached peripherally on the membrane. A mathematical relationship between a pressure differential across opposite sides of the membrane <b>58</b> and the strains as detected by the strain sensors <b>60</b>, <b>62</b> may be derived theoretically, using tools, such as finite element analysis or neural networks, or the relationship may be derived emperically or through statistical analysis, etc.

0050Preferably, the upper chamber <b>54</b> exposed to an upper side of the membrane <b>58</b> has atmospheric pressure therein. The strain sensors <b>60</b>, <b>62</b> are attached on the upper side of the membrane <b>58</b>. Well pressure is admitted into the lower chamber <b>56</b>, where it is in contact with a lower side of the membrane <b>58</b>. Thus, the pressure differential across the membrane <b>58</b> is the difference between well pressure and atmospheric pressure on opposite sides of the membrane.

0051Referring additionally now to <figref idref="DRAWINGS">FIG. 6</figref>, another sensor system <b>70</b> embodying principles of the present invention is representatively illustrated. The sensor system <b>70</b> includes an outer housing <b>72</b> and an inner tubular structure <b>74</b> similar in some respects to the sensor system <b>20</b> shown in FIG. <b>2</b>. An annular space <b>76</b> is positioned between the structure <b>74</b> and the housing <b>72</b>, and two strain sensors <b>78</b>, <b>80</b> are externally attached to the structure in the annular space.

0052The strain sensors <b>78</b>, <b>80</b> are preferably fiber optic sensors similar to the sensors <b>24</b>, <b>26</b>, <b>60</b>, <b>62</b> described above. In this embodiment, the sensors <b>78</b>, <b>80</b> are wrapped multiple times about the structure <b>74</b>, so that a larger magnitude of dimensional change (in the circumferential direction) in the structure is detected by each of the sensors as pressures and temperatures acting on the structure change. Of course, the strain sensors <b>78</b>, <b>80</b> may be otherwise attached to the structure <b>74</b> in keeping with the principles of the invention. For example, the sensors <b>78</b>, <b>80</b> could be attached to the structure <b>74</b> so that they detect axial strain, etc.

0053The structure <b>74</b> is tubular only in a lower portion <b>82</b> thereof. An upper portion <b>84</b> of the structure <b>74</b> is solid. Fluid pressure is admitted into the structure <b>74</b> via an opening <b>85</b> into the interior of the tubular lower portion <b>82</b>. A differential between pressure in the annular space <b>76</b> and the fluid pressure admitted into the opening <b>85</b> acts only across the lower tubular portion <b>82</b> of the structure <b>74</b>. The upper strain sensor <b>78</b>, thus, does not detect strain induced in the structure <b>74</b> by the pressure differential.

0054Instead, the upper strain sensor <b>78</b> detects strain in the structure <b>74</b> induced only by temperature change. The lower strain sensor <b>80</b> detects strain induced both by the pressure differential and by the temperature change. The strain induced by the temperature change in the structure <b>74</b> is preferably detected the same by each of the strain sensors <b>78</b>, <b>80</b> (for example, by using the same type of strain sensor for both of the sensors <b>78</b>, <b>80</b>, mounting the sensors similarly on the structure, etc.). Therefore, the strain in the structure <b>74</b> due only to the pressure differential may be readily determined by finding the difference between the strains detected by the sensors (i.e., strain detected by lower strain sensor <b>80</b>− strain detected by upper strain sensor <b>78</b>= strain due to pressure differential).

0055A predetermined relationship between the pressure differential and the strain due to the pressure differential (e.g., the mathematical relationships described above) may then be used to calculate the pressure differential. However, it should be understood that sensor systems incorporating principles of the invention may be differently configured so that strain sensors thereof detect other strains in a structure.

0056Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to the specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.